Such a method is used, for example, in present-day motor vehicles in order to check the filling level sensor in a fuel tank, the measured change in filling level being compared with a value which is calculated from the amount of fuel which has been consumed and thus has been taken from the tank. To this end, during normal operation of the internal combustion engine which consumes the fuel, a first filling level value is measured and a timer (counter) is then started. A second filling level value is measured after the timer has timed out. While the timer is being started up, a model of the operating behavior of the internal combustion engine is used to determine the amount of fuel consumed. If the change in filling level determined from the difference between the first filling level value and the second filling level value matches the change in filling level derived from the amount of fuel consumed, the filling level sensor is classified as being correctly functional.
DE-8331743-U1 discloses another method for checking the functionality of a filling level sensor that functions as a limit-value transmitter. In that case, compressed air is used to generate an air bubble which pushes the liquid to be measured out of the measurement range of the filling level sensor, with the result that the sensor indicates that the filling level has been undershot.